This invention relates generally to surface acoustic wave devices and more particularly to narrowband surface acoustic wave resonators.
As is known in the art, SAW surface acoustic wave resonators are used in high precision and highly stable, low noise oscillators. The SAW device is used as a frequency stabilizing element in the feedback loop of both electronically controlled oscillators and free running oscillators. Such oscillators are used in radar systems, electronic counter measure systems, as well as, other communication and electrical systems.
In a radar system a low noise, highly stable oscillator is often used to synchronize all the radar signals in phase and frequency. One requirement for advanced radar systems is to increase the capability of the radar to detect small objects at further distances. It can be shown that the detection capability of a radar is related to the system noise which is related to the output power and noise characteristics of the oscillator. If more output power can be extracted from an oscillator without increasing the noise level of the oscillator, then the noise floor of the radar system will be reduced. Reduced system noise will increase the signal to noise ratio of the radar and concomitant therewith the detection capabilities of the radar. Thus, it is desireable to provide higher output powers at low noise levels from SAW-stabilized oscillators to reduce the system floor noise of the radar and increase its detection capabilities.
Generally, in there applications, SAW resonators are used in the feedback loop of the oscillator to stabilize the frequency of oscillations of the feedback loop. A SAW resonator generally includes a surface which supports surface acoustic wave propagation having disposed thereon a pair of spaced interdigitated transducers which are coupled to the surface wave propagating surface. Each interdigitated transducer includes a pair of terminals having a plurality of conductive members, with the conductive members associated with one terminal being interdigitated with the conductive members associated with the other one of the terminals. A pair of reflecting gratings are disposed to confine surface wave propagation as also known.
To provide a SAW resonator which can operate at a relatively high power, it is generally necessary to increase the acoustic aperature of the SAW resonator. The acoustic aperture is increased by increasing the length of the conductive stripes. Increasing the length of the conductive stripes, however, has the effect of increasing the resistance of the stripes which increases the insertion loss of the transducers. Increased insertion loss is generally undesirable. Therefore, in order return the insertion loss of the SAW resonator to that provided previously, it is often necessary to increase the number of conductive members. This results in more conductive members being connected in parallel to provide an effective overall reduction in the resistance of the members and thus returning the transducer to comparable low insertion loss.
Commonly, it is also desired to provide a SAW resonator having a relatively low insertion loss. This is accomplished as indicated above by increasing the number of conductive strips in the transducer. Reducing insertion loss has the effect of lowering the Q of the resonator and likewise degrades noise characteristics of the resonator.
It has been known for some time that high order transverse modes propagate in SAW resonator, as described in a paper by H. Haus entitled "Modes in SAW Grating Resonators", Journal of Applied Physics, Vol. 48, No. 12, December 1977, pg. 49, 55. The above paper indicates that Rayleigh wave type propagation of higher order transverse modes occurs in SAW grated resonators.
Higher order transverse mode propagation includes even order transverse modes and odd order transverse modes. The even order transverse modes are generally not a problem, since the phase of even transverse modes is symmetrically distributed occurs across the acoustic aperture of the device. The phase of odd order transverse modes, however, is a problem since the phase is symmetrically distributed. The acoustic energy stored in the resonant cavity at higher order odd transverse mode has a different spacial distribution than the frequency of the fundamental mode. Further, the resonant frequencies of the higher order transverse modes are higher than the frequency of the fundamental resonant mode.
Such higher order modes have not generally been a problem in SAW resonators when used in standard, low power oscillator applications. This is because in the standard resonator device, the third order transverse mode typically has a magnitude of between about 2-10 dB below the minimum insertion loss of the fundamental mode, and at a frequency of about 50 to 100 ppm higher than the frequency of the fundamental mode. With high power and/or low insertion loss resonators, it has been found that the wider aperatures and extra fingers change the frequency and insertion loss characteristics of the third order transverse mode.
With high power resonators for example, the insertion loss of the third order mode may appear only 1 dB below the insertion loss of the fundamental mode, at a frequency of about 35 ppm above the frequency of the fundamental. The fundamental mode frequency is generally determined by its 1 dB bandwith, that is the frequency range over which the insertion is within 1 dB of the minimum insertion loss of the fundamental mode. Thus, it becomes apparent that with higher power/lower insertion loss resonators, the third order transverse mode may be encompassed within the 1 dB bandwith of the resonator and will significantly decrease the Q of the resonator. This arrangement will degrade the phase noise performance of an oscillator using such a SAW resonator.
Accordingly, the requirement for new high power/low insertion loss resonators have presented a need to address the third order transverse mode and higher odd-order modes in the SAW resonator.